DE 102 37 933 A1 discloses a tray sealer used for producing skin packs in which the product protrudes beyond a tray edge. To this end, the skin film is clamped in position on the sealing-tool upper part and drawn into a dome prior to moving the sealing-tool lower part, together with the tray and the product, into contact with the sealing-tool upper part. The skin film does not come into contact with the product until the skinning process takes place.
EP 2 412 643 A1 shows a thermoform packaging machine with a top-film forming station for sealing a product that protrudes vertically beyond a tray edge with a shrink film and a trough, without the product itself deforming the shrink film by an upward movement prior to the sealing process. The top-film forming station, which is located upstream of the sealing station, ensures that the shrink film is pre-deformed. A clamp chain is arranged on both sides for laterally holding and guiding the deformed shrink film while it is being conveyed from the top-film forming station to the sealing station, and for positioning it precisely onto the trough.
WO 2004/000650 A1 discloses a thermoform packaging machine for producing a shrink pack, where both the bottom film and the top film are shrink films. The thermoform packaging machine comprises a bottom-film and a top-film forming station, in which each film is thermoformed individually prior to the sealing process, so that a product to be packed can be packed in the sealing station. The shrinking effect of the films, through which the films are caused to wrap tightly around the product, is produced in a shrinking apparatus, which is located downstream of the thermoform packaging machine, through introduction of heat, for example, in a hot water bath. A difference between shrink packs and skin packs is to be seen in that in the case of shrink packs the heat for the shrinking process is supplied only after the packaging process, whereas in the case of skin packs the heat is introduced into the skin film already in the sealing station prior to the sealing process. A similar shrink film packaging machine is disclosed in EP 2 412 643 A1.
EP 0 270 208 A1 discloses a thermoform packaging machine for producing a skin pack. A skin film used as a top film and a trough, which has been formed in a bottom film and into which a product has been placed, are conveyed together into a first forming station and are clamped on all sides thereof between a forming-tool lower part and a forming-tool upper part for drawing the skin film then upwards into a dome by means of a vacuum and for heating it there. This process results in a minor deformation so that, when the forming-tool lower part and the forming-tool upper part have been opened, the skin film, while being conveyed together with the trough and the product into a downstream sealing station, will not apply pressure to the product as long as the product does not protrude too far upwards beyond the trough. Due to the heat input, the skin film already exhibits an elasticity which is suitable for allowing the skin film to be drawn, in the sealing station after the formation of a chamber through a sealing-tool lower part and a sealing-tool upper part, into a dome by means of a vacuum, the dome comprising a much larger (four times larger) hollow than the dome in the forming-tool upper part. Subsequently, the skin film is heated to an elevated temperature in the dome for producing then, by means of a vacuum between the skin film and the trough, a skin pack in the case of which the skin film wraps tightly around the product and the inner surfaces of the trough, thus sealing the product in an air-tight manner. A larger deformation of the skin film in the forming station is not provided.
Products protruding vertically beyond the trough edge entail the risk of wrinkle formation in the clamping area during closing of the sealing-tool lower part and the sealing-tool upper part, and this may lead to faulty sealing and leakiness later on. In order to reduce or avoid such wrinkle formation, also the lateral distance from the product to the trough edge has hitherto been increased, when there is an increase in product protrusions. The resultant packages thus have a large trough area with an upwardly protruding product. These packages are, on the one hand, not attractive and, on the other hand, they necessitate the use of a larger amount of top film and bottom film, which, in turn, results in an increase in packaging costs and simultaneously in a reduction of the packaging performance of the thermoform packaging machine, since the number of products that can be processed during one work cycle decreases. In thermoform packaging machines with a multi-lane or multi-row format, the risk of wrinkle formation increases still further.